Heat-sensitive recording includes advantages such as that (1) development is unnecessary, (2) when paper is selected as a support, the paper quality may be close to that of an ordinary paper, (3) handling is easy, (4) the colored images of high density can be provided, (5) a recording apparatus is inexpensive because of its simple structure, and (6) no noise is generated in recording. Thus, recently, use of heat-sensitive recording methods has rapidly spread in the fields of facsimile equipment and printers. As such a heat-sensitive recording material, mainly heat-sensitive recording material of the leuco color forming type which is excellent in its color forming density and its color forming speed has been used.
Generally, thus far, heat-sensitive recording has been mainly applied to character recording. Recently, however, the desire for picture recordings having gradation, such as a hard copy of a television picture, has been increasing. In general, there are two methods used for the expression of gradation, one being such that the recording density is fixed while a recording area is varied, and the other being such that a recording area is fixed while recording density is varied. The latter is superior to the former because not only is the resolving power low, but also the capability for expression of halftone by the former is inherently limited. The conventional heat-sensitive paper, however, is generally more adaptable for the former method, and does not have enough aptitude for the latter method. This is because, in the latter method, printing of half density is required with respect to the heat-sensitive paper. That is, the change in response is required to be limited with respect to the change in applied energy (that is, the rate of change is made to be gradual).
Conventional heat-sensitive recording paper of the leuco coloring type is, however, such that a leuco dye and a color developer of a phenol group are finely dispersed in size of micron order and mixed with each other and then applied onto a paper support and that the color components thereof are fused to be mixed with each other to form colors when heated by a thermal head. It is not easy to provide a difference in a temperature characteristic between particles or applied layer portions.
In conventional heat-sensitive paper, the rate of change in coloring density in response to applied energy (that is, the quantity of heat applied) is large and therefore low in recording with suitable gradation.
Various kinds of new functions have been required in such recording fields, and, for example, facsimile equipment for transmitting color documents, color printers, etc., have been developed. For these printing systems, an ink jet system, a thermal transfer system, or the like, is employed. These systems, however, have disadvantages such as that an ink nozzle may become clogged. In a heat-sensitive system, heat-sensitive paper capable of forming multicolors is known. For example, there are application of diazosulfonate, a combination of diazosulfonate and a leuco coloring system, etc. They are compounds producing a coloring reaction and require to have a difference in thermal responsiveness, so that color elements capable of being used are extremely limited and there are many restrictions in design and production of the recording materials.